Resonant circuits which are excited to resonate at a predetermined resonant frequency which is conventionally at 8.2 MHz are widely accepted to protect articles against pilferage in department stores. Frequently the circuits are an integral part of adhesive labels or cardboard tags which are affixed to the articles to be maintained under surveillance. Typically, the department store has an electronic surveillance system installed in the exit area, which detects the resonant circuits and produces an alarm when a protected article passes through a surveillance zone in an unauthorized manner. The resonant circuit is deactivated when a customer has paid the merchandise. This prevents an alarm being produced once an article has been rightly acquired by purchase, passing through the surveillance zone subsequently.
The deactivation systems which are frequently installed in the checkout areas generate a resonant signal of a higher amplitude than it is produced in the surveillance systems. A resonant label is normally deactivated with a field strength greater than 1.5 Ampere-turns per meter, A/m.
A variety of deactivating mechanisms for resonant circuits are known in the art. They involve either destroying the insulation between two opposing conductive tracks, producing a short circuit, or subjecting a length of conductive track to overload and causing it to melt, thereby interrupting the circuit path. As a consequence of deactivation, the resonant properties of the resonant circuit, that is, the resonant frequency and/or the "Q" factor are modified so severely that the resonant label stops being detected by the surveillance system.
There is a risk that the deactivated resonant circuit may be reactivated inadvertently by mechanical manipulation including, for example, folding, packaging and transporting the merchandise, or bending the label and hence the resonant circuit. Any accidental reactivation of a resonant circuit which is affixed to an article rightly acquired by purchase may then produce an alarm leading to embarrassment both for the customer and for the department store.
So far no state of the art has become known which concerns itself with the problem of diminishing the risk of an accidental reactivation of resonant labels that are already deactivated. With regard to the deactivation of resonant labels, different methods have been described in the art. In U.S. Pat. No. 4,876,555 and its corresponding European Patent, EP 0 285 559 B1 a it is proposed to use a needle to produce a hole in the insulating layer between two opposite capacitor surfaces. This results in a fault-free and permanent deactivation mechanism.
U.S. Pat. No. 5,187,466 describes likewise a method for generating a deactivatable resonant circuit by means of a short circuit that cannot be destroyed under normal circumstances.
As regards the first mentioned U.S. Pat. No. 4,876,555 and its corresponding European Patent, EP 0 285 559 B1, it should be noted that the resonant circuit therein disclosed includes capacitor plates which are disposed on either side of a dielectric material. The dielectric layer arranged between the two capacitor plates has a through hole.
In U.S. Pat. No. 5,187,466 referred to in the foregoing, a method is described which is applied to a resonant circuit having capacitor plates on either side of a dielectric, and in which the capacitor plates are first short-circuited and the short circuit is melted later by the application of electrical energy.
Still further important techniques in the field of the de-activation of resonant labels are known which however do not concern themselves with the reduction of the risk of an accidental reactivation. A patent family extending in this direction comprises, among others, European Patent, EP 0 181 327 B1, U.S. Pat. No. 4,567,473 and U.S. Pat. No. 4,498,076. The resonant label of the present invention which is described in these patents is composed of the following components: a substrate material serving as a dielectric, capacitor plates on either side of the planar dielectric substrate material, a deactivation zone and a resonant circuit which is disposed on the dielectric material. Heretofore the state of the art has not indicated any provisions that would prevent an undesirable reactivation after deactivation has taken place successfully.